In recent years, an increasing number of light sources for general illumination such as an incandescent light bulb and fluorescent lights have been replaced with white light-emitting diodes (LEDs). Other than the above light sources for general illumination, there are also illumination light sources such as recessed lights for a shop, headlights of a car and the like, and projector light sources. Such light sources include halogen lamps, high-pressure mercury lamps, metal halide lamps and the like. Of these, high intensity discharge lamps, such as a high-pressure mercury lamp or a metal halide lamp, use arc discharge, so that highly directive light can be emitted efficiently and at high power. However, such high intensity discharge lamps have the following disadvantages: stabilization of light output after lighting takes time; environmental burden is large due to inclusion of mercury; and the time it takes until luminance is halved, which is defined as the end of the life, is short.
In order to overcome such disadvantages, recent years have seen intensive development of light-emitting devices which include semiconductor light-emitting elements, such as LEDs or semiconductor lasers, as light sources or excitation light sources. Various structures of the light-emitting device including the semiconductor light-emitting elements are available depending on the intended use. One example is a light-emitting device which varies emission wavelengths in the visible light range (430 nm to 660 nm) by including different semiconductor materials or compositions. Another example is a light-emitting device which varies emission wavelengths or emission spectra by including phosphors in combination.
For example, patent literature (PTL) 1 and 2 disclose a white LED which includes an LED in combination with a phosphor having a host material that is aluminate or orthosilicate and containing europium (Eu) as an activator.
PTL 3 discloses another example of phosphor materials. PTL 3 discloses a white LED including a blue LED, which emits blue light of a wavelength ranging from 430 nm to 460 nm, in combination with a so-called YAG phosphor having a host material that is (Y,Gd)3(Al, Ga)5O12 and containing cerium (Ce) as an activator. The white LED including the YAG phosphor exhibits high wall plug efficiency (WPE). The YAG phosphor, however, has properties in which the absorption spectrum has a peak in a wavelength ranging from 440 nm to 450 nm. Such properties result in decreased conversion efficiency of light of a wavelength different from the wavelength of the blue light, or result in the emission spectrum peak ranging from 530 nm to 590 nm (greenish yellow to orange). Hence, the spectrum of the white LED using the above materials is so-called quasi-white that is obtained by mixing the blue light directly emitted from the blue LED and the yellow light emitted from the phosphor.
Light-emitting devices such as the white LEDs including the semiconductor light-emitting elements have been widely used for general illumination as described above. On the other hand, intensive development of such light-emitting devices has also been made for various illumination purposes, by improving properties other than the conversion efficiency of emitted light from the light-emitting devices. In particular, replacement of the light-emitting devices including the semiconductor light-emitting elements is expected to progress further by improving the following properties. The properties are: directivity of a light-emitting unit, such as Etendue; color reproducibility (correctness in chromaticity coordinates) of white light; and conversion efficiency, color purity, and speckle properties of three primary colors (blue, green, and red).
Of those properties, relative to a light-emitting device used for display applications, there is a strong demand particularly for color reproducibility, such as a demand for accuracy of 1/100 in chromaticity coordinates. In response to such a demand, for example, PTL 4 discloses a light-emitting device which includes semiconductor lasers in combination which respectively emit blue light, green light, and red light. PTL 5 discloses a light-emitting device which includes semiconductor lasers which emit blue light, a Y3(Al,Ga)5O12 phosphor (green phosphor), and a CASN phosphor (red phosphor) in combination. However, emitted light of three primary colors from these light-emitting devices partially or entirely includes light directly emitted from the semiconductor lasers. Hence, there are a safety problem that occurs when the emitted light directly enters an eye of a user, and a problem of decrease in image quality caused by speckle noise that occurs due to coherency.
In order to solve such problems, PTL 6 discloses a light-emitting device which emits light of three primary colors entirely from phosphors. The light-emitting device includes a semiconductor light-emitting element which emits ultraviolet light, in combination with a circular plate provided with a red phosphor layer, a green phosphor layer, and a blue phosphor layer. Referring to FIG. 20, a description is given below of a conventional light-emitting device disclosed in PTL 6.
As FIG. 20 illustrates, a conventional light-emitting device includes: light-emitting diodes 1003 which emit ultraviolet light; and a color wheel 1004 provided with three phosphor layers respectively including a red phosphor, a green phosphor, and a blue phosphor in partitioned regions. The color wheel 1004 is rotated, so that the color of light emitted from the light-emitting diodes 1003 is sequentially converted, for example, in the order of red, green and then blue. The light-emitting device is driven such that white light is emitted in time-averaged observation. In this structure, (Sr,Ca,Ba,Mg)10(PO4)6C12:Eu or (Ba,Mg)Al10O17:Eu is used as a blue phosphor, ZnS:Cu, Al or (Ba,Mg)Al10O17:(Eu,Mn) is used as a green phosphor, and Y2O2S:Eu is used as a red phosphor.
Moreover, PTL 7 discloses that when an LED is used as a light source, luminous efficacy can be optimized by making the Eu concentration in an Eu-activated BaMgAl10O17 phosphor 20 mol % or greater.